Many forms of cryogenic freezers have been designed for the use of such cryogenic refrigerants as liquid nitrogen and liquid carbon dioxide. Since liquid nitrogen remains in liquid phase during expansion through a nozzle into the freezer, and thereafter vaporizes into cold gas upon contact with the relatively warm product, it is common to utilize a spray header and a plurality of gaseous pre-cooling zones as disclosed in U.S. Pat. No. RE 28,712, U.S. Pat. Nos. 3,403,527, and 3,813,895. Alternatively, some freezers such as disclosed in U.S. Pat. No. 3,611,745 have employed indirect heat exchange of the liquid nitrogen with the product, and have circulated the vaporized nitrogen gas as a protective atmosphere in large volume freezing chambers using a plurality of circulating fans.
In the case of liquid carbon dioxide, the expansion of the liquid refrigerant through the injection nozzle causes the liquid to vaporize into a mixture of gas and solid particles. Some prior freezers, such as that disclosed in U.S. Pat. No. 4,086,784, spray the carbon dioxide snow directly on the product and circulate the gas with a plurality of axial flow fans. Other freezers, such as that disclosed in U.S. Pat. No. 3,818,719, inject the cryogenic refrigerant into the discharge of a blower and circulate the gas with plurality of fans. However, these designs require the movement of large volumes of gas which requires significant amounts of fan energy. This results in significant amounts of undesirable heat input into the freezer.
Other freezer designs, such as disclosed in U.S. Pat. Nos. 3,672,181, 3,677,167 and 3,708,995 have ulilized other arrangements of fans and blowers to circulate mixtures of gaseous and solid carbon dioxide in contact with products to be frozen. However, the velocities of the gas-solid mixtures have been relatively low, and a plurality of fans or blowers are required to circulate the large volumes of the refrigerant mixture which results in an undesirable heat input to the freezer. Also, problems have been encountered with the build-up of carbon dioxide snow such that the freezers must be operated at temperatures significantly warmer than the sublimation temperature of the CO.sub.2.